Section 6 at Grade Intersections
Total Page：16
File Type：pdf, Size：1020Kb
BDC07MR-05 Section 6 At Grade Intersections 6.1 General Most highways intersect at grade. To minimize the resulting conflicts and to provide adequately for the anticipated crossings and turning movements, the geometric design of the intersection at grade must be given careful consideration. Although intersections have many common factors, they are not subject to a set treatment, and must be looked upon on a case by case basis. In varying degrees, four basic factors enter into the design of an intersection. These factors are traffic, physical, economic, and human. Traffic factors to be considered include: Possible and practical capacities Turning movements Size and operating characteristics of vehicles Control of movements at points of intersection Vehicle speeds Pedestrian Activity and movements Bicycle Activity and movements Bicycle operating space Transit operations Crash experience Physical factors which control intersection design and application of channelization are: Topography Abutting land use Geometric features of the intersecting roadways Traffic control devices Safety features Economic factors, which are important and often controlling, include: Cost of improvements Effect on abutting businesses where channelization restricts or prohibits certain vehicular movements within the intersection area Human factors such as: Driving, pedestrian, and bicyclist habits Ability of drivers, pedestrians, and bicyclists to make decisions Effect of surprise events Decision and reaction times NaturalSuperseded paths of movements must be considered Types of pedestrians An intersection may be extremely simple, or highly developed depending on the proper evaluation of the foregoing factors. In the redesign of an existing intersection, standards sometimes must be compromised due to the high cost of existing development or to the necessity of meeting rigid physical controls. In the design of a new intersection, NJDOT Design Manual – Roadway 6-1 At Grade Intersections BDC07MR-05 however, such controls frequently can be avoided by a shift in line or grade of one or both of the intersecting highways. For further general discussion and details, see AASHTO – A Policy on Geometric Design of Highways and Streets. 6.2 General Design Considerations 6.2.1 Capacity Analysis Capacity analysis is one of the most important considerations in the design of intersections. This is especially true in the design of at-grade intersections on urban streets and highways. Optimum capacities can be obtained when intersections include auxiliary lanes, proper use of channelization, and traffic control devices. Where these techniques are under consideration, it is necessary to consider pedestrian and bicycle safety and level of service. Reference is made to the Highway Capacity Manual, Transportation Research Board, for procedures in performing capacity computations. 6.2.2 Spacing The spacing of intersections on major arterials is important to the capacity and safety of the roadway. In urban areas, the capacity of the arterial is determined by the capacity of the signalized intersections along the roadway. Ideally, signalized intersections should be located no closer than 1200 feet apart. In rural areas, the minimum spacing of intersections should be one half mile. 6.2.3 Alignment and Profile Intersections are points of conflict between vehicles, bicycles, and pedestrians. The alignment and grade of the intersecting roads should permit drivers to discern and perform readily the maneuvers necessary to pass through the intersection safely and with minimum interference between vehicles. To these ends, the horizontal alignment should be as straight as possible and gradient as flat as practical. The sight distance should be equal to or greater than the minimum values for the specific intersection conditions. Sight distance is discussed later in this section. 1. Alignment Regardless of the type of intersection, intersecting highways should meet at or nearly at right angles. Roads intersecting at acute angles require extensive turning roadway areas. Intersection angles less than 60 degrees normally warrant realignment closer to 90 degrees. Intersections on sharp curves should be avoided wherever possible because the superelevation and widening of pavements on curves complicate the intersection design. Furthermore, since traffic stripes are not normally carried through the intersection, there is no visual reference for the guidance of the driver through the intersection curve during adverse weather and visibility conditions. 2. Profile Combinations of profile lines that make vehicle control difficult should be avoided. Substantial grade changes should be avoided at intersections, although it is not always Supersededfeasible to do so. Adequate sight distance should be provided along both highways and across corners, even where one or both intersecting highways are on vertical curves. The grades of intersecting highways should be as flat as practical on those sections that are to be used for storage space for stopped vehicles. A minimum storage space for 2 vehicles, approximately 50 feet, should be provided for minor streets where stop NJDOT Design Manual – Roadway 6-2 At Grade Intersections BDC07MR-05 sign control is employed and the approach grade is up towards the intersection. Such slopes should desirably be less than one percent and no more than 3 percent. The profile lines and cross sections on the intersection legs should be adjusted for a distance back from the intersection proper to provide a smooth junction and proper drainage. Normally, the profile line of the major highway should be carried through the intersection, and that of the cross road adjusted to it. Intersections with a minor road crossing a multi-lane divided highway with narrow median and superelevated curve should be avoided whenever possible because of the difficulty in adjusting grades to provide a suitable crossing. Profile lines of separate turning roadways should be designed to fit the cross slopes and longitudinal grades of the intersection legs. As a rule, the horizontal and vertical alignments are subject to greater restrictions at or near intersecting roads than on the open road. Their combination at or near the intersection must produce traffic lanes that are clearly visible to the motorists, bicyclists, and pedestrians at all times and definitely understandable for any desired direction of travel, free from sudden appearance of potential hazards, and consistent with the portions of the highway just traveled. 6.2.4 Cross Section The cross section of the pavement surface within an intersection should be reviewed on a case-by-case basis. The development of the centerline profiles and edge of pavement profiles should flow smoothly through the intersection. 6.3 Sight Distance 6.3.1 General There must be unobstructed sight along both roads at an intersection and across their included corner for distances sufficient to allow the operators of vehicles approaching the intersection or stopped at the intersection to observe pedestrians and cyclists and carry out whatever maneuvers may be required to negotiate the intersection. It is of equal importance that pedestrians be able to view and react to potential conflicts with vehicles. Any object within the sight triangle high enough above the elevation of the adjacent roadways to constitute a sight obstruction should be removed or lowered. Such objects include but are not limited to cut slopes, hedges, bushes, tall crops, signs, buildings, parked vehicles, etc. Also check the vertical curve on the highway to see if it obscures the line of sight from the driver's eye (3.5 feet above the road) to the approaching vehicle (3.5 feet above the road) as per the sight distance determined in the next three sections. 6.3.2 Sight Distance for Bicycle Facilities In general the sight distance required to see a bicycle is no greater than that to see a vehicle, so bicycle sight distance need not be calculated at intersections also used by vehicles. At locations where a separated bicycle facility crosses the roadway, or elsewhere where cyclists may enter or cross the roadway independent of vehicles, appropriateSuperseded sight distance should be provided. Vehicles parked near crosswalks create sight distance problems, and for this reason New Jersey State Statutes prohibit motor vehicle parking “within 25 feet of the nearest crosswalk or side line of a street or intersecting highway, except at alleys,” and within 50 feet of a stop sign (Title 39:4-138). These relationships also apply to other locations where pedestrians are likely to cross (mid-block crosswalks, T-intersections, gaps in median barriers). NJDOT Design Manual – Roadway 6-3 At Grade Intersections BDC07MR-05 The parking setback distance can be reduced in locations where curb extensions have been provided to reduce crossing distance and increase the visibility of pedestrians as long as the provisions of Title 39 are also met. 6.3.3 Stop Control on Cross Street Intersection designs should provide sufficient sight distances to avoid potential conflicts between vehicles turning onto or crossing a highway from a stopped position and vehicles on the through highway operating at the design speed. As a minimum stopping sight distance must be provided. However, to enhance traffic operations, the recommended sight distance along the major roadway from Figure 6-A for various design vehicles to turn left, right or cross should be provided. Where the median width on a divided highway is 6 feet or greater than the vehicle length, the crossing can be accomplished in 2 steps. The vehicle crosses the first pavement, stops within the median opening, and proceeds when a safe gap in traffic occurs to cross the second pavement. However, when the median width is less than that of a vehicle, the crossing must be made in one step and the median must be included as part of the roadway width (w). 6.3.4 Yield Control When an intersection is controlled by a yield sign, the sight triangle is governed by the design speed on the main highway and that of the approach highway or ramp. Suggested approach speeds on the yield controlled approach are 15 mph for urban conditions and 20 mph to 25 mph for rural conditions.